The present invention is directed to the production of alkoxysilanes and alkoxy orthosilicates by reacting silicon metal with alcohols in the presence of a suitable catalyst.
Alkoxysilanes are useful chemicals for the synthesis of organosilanes used as silane-coupling reagents. Trimethoxysilane (HSi(OCH33)33), for example, can be synthesized directly by reaction of silicon metal with methanol in the presence of copper(I) chloride as a catalyst. Pioneering work by E. Rochow built the direct reaction of silicon with alkyl chlorides into the silicone industry. Rochow discovered that a mixture of elemental copper in silicon provided optimum conditions for reactions in gas fluidized reactors. Later work by the Japanese involved liquid phase reactions using copper chloride catalyst in an aromatic solvent at high temperatures.
Depending upon the reaction conditions, pretreatment conditions, and the particular catalyst chosen, the yield of the desired trialkoxysilane can vary widely. Tetraalkyl orthosilicate is a common by-product (and also a valuable by-product with sufficient commercial value), formed either directly from the reaction of elemental silicon and alcohol, or from the secondary reaction of trialkoxysilane and alcohol. Depending upon the identity of the particular alcohol used in the reaction, alcohol reduction, dehydration and/or dehydrogenation side reactions also may be problematic. Where triethoxysilane is the desired end-product, thermal degradation of the ethanol reactant can lead to olefins and their incorporation into the silane reaction.
U.S. Pat. No. 5,728,858 discloses a direct process for producing trialkoxysilanes in which silicon metal is slurried in a thermally stable solvent in the presence of a halogen-free catalyst precursor. The catalyst precursor includes copper, at least a part of which is not in the copper(0) state and is reducible to the copper(0) state. The copper(0) is then fully reduced to generate a catalyst for the reaction of the silicon metal with an alcohol, and the reaction is carried out. Suitable thermally stable solvents disclosed include polyaromatic and alkylated aromatic compounds. However, previous documents claim aromatics are critical but do not clearly describe chemistry of their role in their process. Using simpler chemical structures can avoid problems in process improvement, control and environmental management.
It therefore would be desirable to provide a process for the production of alkoxysilanes and or alkyl orthosilicates from silicon metal and alcohol in high yield that does not suffer from the drawbacks of the prior art.
It further would be desirable to provide a process for the production of alkoxysilanes and/or alkyl orthosilicates from silicon metal and alcohol that eliminates the pre-reduction of copper to form the catalyst.
It further would be desirable to provide a process for the production of alkoxysilanes and/or alkyl orthosilicates using a liquid phase reaction without an exotic solvent and into which a catalyst could be introduced in order to control the reaction rate.
It still further would be desirable to provide an improved catalyst for the reaction of silicon metal with alcohol to produce alkoxysilanes and/or alkyl orthosilicates in high yield.
Other objects and advantages of the present invention will be made apparent by the following description and examples.
The problems of the prior art have been overcome by the present invention, which provides a process for the production of alkoxysilanes of the formula HSi(OR)3, where R is an alkyl group containing 1 to 6 carbon atoms, and/or alkyl orthosilicates of the formula Si(OR)4, where R is an alkyl group containing 1 to 6 carbon atoms. In a preferred embodiment, the process comprises a slurry reaction wherein silicon metal is reacted with an alcohol in a suitable solvent in the presence of a cupric bis(diorganophosphate) catalyst. A polymeric form of ethyl orthosilicate, a by-product of the reaction, is the preferred solvent. The production of triethoxysilane and tetraethyl orthosilicate is preferred, with triethoxysilane being particularly preferred. Auxiliary reduction with hydrogen or other reducing agents is not required. Judicious addition of catalyst and solvent during the reaction can sustain reaction rate and selectivity.
The process of the present invention is based upon the following chemical reactions:
Si+3ROH+catalyst xe2x86x92HSi(OR)3+H2
wherein R is an alkyl group of 1 to 6 carbon atoms, preferably 1 to 2 carbon atoms, most preferably 2 carbon atoms. The major by-product of this reaction forms the tetraalkyl orthosilicate:
Si+4ROH+catalyst xe2x86x92Si(OR)4+2H2
In addition, the trialkoxysilane can further react in the presence of sodium hydroxide as follows:
HSi(OR)3+H2O xe2x86x92(EtO)3SiOH+H2
The process conditions and catalyst of the present invention favor the production of trialkoxysilane.
In accordance with the present invention, the liquid reaction medium is an organosilicate, preferably alkyl orthosilicate or alkyl orthosilicate polymer. Most preferably the alkyl group is ethyl. Advantageously, alkyl silicate or derivatives thereof are by-products of the reaction to trialkoxysilane. Suitable derivatives are of the formula Et(OSi(OEt)2)nOEt, where n is from 1 to 10, preferably 2 to 5. The most preferred medium is a condensation polymer of tetraethyl orthosilicate corresponding analytically to the pentamer as discussed in further detail below. Tetraethyl orthosilicate itself also can be used. The reaction medium can periodically cleaned such as by extraction and distillation.
More specifically, the basic composition unit of the solvent is tetraethyl orthosilicate or TEOS ((EtO)4Si) commercially known as xe2x80x9cEthyl Silicate Purexe2x80x9d when distilled, or xe2x80x9cEthyl Silicate Condensedxe2x80x9d when obtained without further purification from the manufacturing process. This composition contains 28% SiO2. Ethyl Silicate-40 or xe2x80x9cES-40xe2x80x9d is the major commercial form of ethyl silicate, and contains 40% SiO2. It is easily made from xe2x80x9cethyl Silicate Condensedxe2x80x9d by adding water and a small amount of hydrochloric acid, then distilling the appropriate amount of ethanol. This reaction not only forms dimers and trimers, but a complex permutation of structures in three dimensions. Data demonstrate that the dimer boils at 235xc2x0, whereas the xe2x80x9cmonomerxe2x80x9d TEOS boils at 170xc2x0 C. Higher molecular weight polymers of TEOS boil above 250xc2x0. Thus, the dimer and higher xe2x80x9ccondensation polymersxe2x80x9d of TEOS are well suited for the instant process since they will not boil off with the products TES and TEOS.
By SiO2 content, ES-40 is calculated to average 5 units of monomer, but by analysis commercial ES-40 contains about 25% TEOS. The remaining 75% is polymer ranging from dimer to diverse three dimensional structures containing up to a dozen TEOS units. The less volatile portion of ES-40 ideally remains in the reactor and provides a suitable medium for reacting silicon with ethanol in the presence of catalyst.
The higher molecular weight polymers of TEOS can gel upon prolonged heating and reaction with impurities (especially water) in reactants. ES-40 holds up without significant gelation when held below 220xc2x0 C. for 24 hours, which is sufficient to process a charge of silicon, after which the solvent medium can be re-worked by removing un-reacted silicon and less soluble components formed in the reaction medium. The re-worked medium then can be recycled to the next batch.
Thus the reaction medium can be the dimer, the trimer, higher polymeric organosilicates formed by the reaction of TEOS and TES by water and heating, or mixtures thereof.
The elemental silicon used as a reactant in the process of the present invention is not particularly limited. Suitable sources include commercially available grades of silicon, in particulate or powder form. Purities of commercial grade silicon are in the range of about 80% to about 99% by weight, with particle sized from about 50 to about 100 5 xcexcm. Dry milling silicon along with the catalyst, such as with a vibrating mill or preferably a ball mill, can improve the reaction and is preferred. The amount of silicon used is a function of the amount of solvent. Preferably the amount of silicon used is in a weight ratio of about 1:5 to 2:1 to the amount of solvent.
Suitable alcohols useful as a raw material in the process of the present invention include alkyl alcohols, preferably those having 1 to 6 carbon atoms, more preferably 1 to 2 carbon atoms. Those skilled in the art will appreciate that the particular alcohol chosen, and more specifically, the particular alkyl alcohol chosen, will depend in part on the desired final product. Exemplary alcohols include methyl alcohol, ethyl alcohol, n-propyl alcohol, iso-propyl alcohol, n-butyl alcohol, iso-butyl alcohol, amyl alcohol, mixtures thereof, etc. Methyl and ethyl alcohol are particularly preferred, with ethyl alcohol being most preferred.
The preferred catalyst in accordance with the present invention is a metal organophosphate soluble in the reaction medium and demonstrating sufficient activity at relatively low temperatures. Maintaining the reaction temperature as low as possible reduces or eliminates difficulties associated with thermal dehydration of ethanol, which deleteriously can lead to the formation of olefins and their incorporation into the silane reaction. Olefin formation is generally undesirable due to the difficulty in separating carbosilanes produced by reaction of olefins with silicon from the desired product (e.g., ethyl triethoxysilane formed by reaction of ethylene with silane intermediate). The amount of catalyst used is an amount effective to catalyze the reaction. Generally an effective amount ranges from about 0.1 to about 0.5 parts by weight catalyst per part by weight of the silicon. Those skilled in the art will appreciate that the catalyst:silicon ratio leading to the most efficient reaction depends in part on the surface area of the silicon, with most finely divided silicon accommodating more catalyst.
Most preferably the catalyst is the copper salt of diethylphosphoric acid, Cu((O)P(OEt)2)2. This catalyst has been found to exhibit excellent solubility in the reaction medium and in the alcohol raw material, leave residues compatible with triethylsilicate chemistry, and have a unique ability to interact with silicon surfaces. The solubility of the catalyst in the reaction medium is particularly surprising, in view of the known fact that ionic salts and soaps are sparingly soluble in non-ionic solvents and in orthosilicates. The solubility of the catalyst in alcohol, particularly in ethanol, allows the catalyst to be added with feed if desired. Catalyst also can be added by ball milling with silicon or by direct addition, for example. Ball milling with silicon ensures contact between the catalyst and the silicon before reaction with ethanol, which can be advantageous. Contact during reaction with ethanol also can be advantageous.
The cupric bis(diethyl phosphate) can be prepared by several methods apparent to those skilled in the art. The procedure described by C. M. Mikulski et al. published in Z. Anorg. Allg. Chem. 1974, 403(2), 200-210, the disclosure of which is hereby incorporated by reference, is suitable for preparing the catalyst. That procedure heat cupric chloride with triethyl phosphate. The reaction displaced chloride while forming the diethylphospate needed to build the desired compound.
Other copper catalysts effective for catalyzing the reaction and that have appreciable solubility in the orthosilicate solvent (about 1% or more by weight of compound in the solvent) are within the scope of the present invention. Salts containing polymeric ether in their structure and other organophosphates containing haloalkyl groups, such as cupric bis(2-chloroethyl)phosphate, are examples.
Suitable reaction temperatures are between about 120xc2x0 C. and about 250xc2x0 C., more preferably between about 150xc2x0 C. and about 220xc2x0 C., most preferably between about 160xc2x0 C. and about 200xc2x0 C. Suitable reaction pressures are from about 0.1 to 10 atmospheres, preferably from about 0.5 to 1 atmosphere. Reaction in a conventional stirred tank reactor has been found to be suitable, thereby utilizing conventional equipment and minimizing capital expense for specially designed equipment. Positioning the point of feed of the alcohol to encourage multi-point sparging of alcohol into the reactor is preferred. Removal of significant heat of reaction in larger reactors can be accomplished by pumping the reaction slurry through an external heat exchanger. Silicon can be added with catalyst to make-up silicon used up by the reaction. Preferably the reaction conditions and solvent:silicon ratio are maintained constant throughout the reaction to enhance reactivity and selectivity. Localized concentration and holdup of alcohol should be avoided by utilizing rapid mixing and by eliminating pockets were alcohol can accumulate.
The order of addition of reactants is not particularly limited. For example, the alcohol can be added to a slurry of fine silicon metal suspended in ethyl orthosilicate oligomer with the catalyst. The solubility of the catalyst in the solvent allows the catalyst to be added with the feed. Additional catalyst can be added during the reaction to increase the reaction rate or to control the reaction. Alternatively, the catalyst can be milled with the silicon or can be added directly to the reaction.
If activation of silicon is desired, the medium can be tetraethyl orthosilicate or an ethyl orthosilicate oligomer. Activation of silicon can be carried out simply by heating catalyst with silicon in the medium prior to alcohol addition. It is believed that the activation process occurs spontaneously while addint alcohol to the silicon slurry in the organosilicate solvent, and is essentially complete while waiting an hour or two after the reation has attained 150xc2x0 C. or higher.
Trialkoxysilane product should be removed from the reaction vessel as quickly as possible. If removal results in significant depletion of solvent, fresh solvent can be added back to the reactor. Since the solvent is a reaction by-product, additional solvent also can be added by partially condensing the solvent from reaction vapor with a distillation column. Depletion of solvent can be minimized by operating at lower temperature or by using higher boiling fractions of the solvent.
Partial condensation can carry undesirable amounts of triethoxysilane back into the reaction, where triethoxysilane can over-react to tetraethoxy orthosilicate, or can disturb the catalytic process. This can be avoided by stripping triethoxysilane from the orthosilicate before recycle.
The following examples are for illustration and are not to be construed as limitations on the present invention.
Unless otherwise indicated, the following standard procedure was used.
Into a 500 ml. spherical flask with three necks fitted with an electric heating mantle whose heat output is controlled by a transformer and a mechanical stirrer whose speed is maintained to suspend the silicon, is charged with 20 g standard silicon powder (200 mesh, 98+% purity dry powder) and 100 g organosilicate fluid (TEOS or ES-40). Catalyst is added to the slurry.
One neck of the flask is attached to a water cooled condenser whose inlet drains into a trap with a stopcock outlet. Thermometers are inserted into the flask and below the condenser to measure slurry and vapor temperatures. The system is sealed to vent by-product hydrogen gas only through the condenser outlet.
The slurry is pre-heated to a target temperature range and held for a determined interval (usually 1 hour above 160xc2x0 C.), then ethanol (below 10 ppm water) addition is started beneath the surface of the slurry and m maintained at a standard rate (about 20 cc/hr). The temperature profile is maintained by adjusting the mantle voltage. Condensate is collected in the trap and removed at specific time intervals for weighing and analysis.
Samples of distillate are analyzed for triethyl silane (TES) by measurement of hydrogen released by hydrolysis. Hydrogen evolutoin is also measured during the specific time intervals. Net hydrogen volume difference between amount evolved and amount produced by TES reaction is used to calculate the amount of TEOS produced during the interval.